A physical box or digital equivalent where people can submit suggestions, often found in workplaces or public facilities.
会社の廊下に提案箱が設置されている。
A suggestion box is installed in the company hallway.
提案箱に改善案を入れた。
I put an improvement proposal into the suggestion box.
Compound of 提案 (ていあん, 'proposal') and 箱 (はこ, 'box'), literally 'proposal box'.